Typical induction motor designs have a rotor including a plurality of laminated sheets (laminations) and a shaft attached to the rotor. Interaction between magnetic fields created by the rotor and a stator surrounding the rotor causes torque to be created in the rotor. This torque rotates the rotor, thereby also rotating the attached shaft. Therefore it is desirable that the connection between the shaft and the rotor be sufficiently strong to allow transmission of the torque from the rotor to the shaft. An additional requirement in attaching the shaft to the rotor core is that the rotor should be strong enough, both torsionally and laterally, to withstand any load, such as a belt load, to which it will be subjected. Further, the rotor should be strong enough so that parts of it do not crack, break, or otherwise fail while it is spinning, particularly at higher speeds.
Conventionally, shafts have been attached to rotor cores through a central hole in the laminations making up the rotor core. The shaft is attached to the rotor core by means such as heat shrinking, welding, and/or a key on the shaft that fits into a slot in the laminations, thereby allowing the core to transmit torque to the shaft.
The conventional method of attaching the shaft to the core through a central hole has the drawback of inducing a stress concentration in the laminations. It is well known that the presence of even a very small hole at the axis of a rotating disc will cause the stress of the material bounding the hole to double when compared with the stress at the center of a solid disk. See, e.g., Adel S. Saada, Elasticity: Theory and Applications, p. 337 (1983). Thus the central hole in the laminations leads to high tangential stresses in the lamination material in the vicinity of the hole. These high stresses can lead to failure of the rotor, due to radial cracks in the laminations.
In view of the above, it would be desirable to attach a shaft to a rotor core of a dynamoelectric machine without disturbing the lamination material in the vicinity of the axis of the rotor.